1. Field of the Invention
The present invention relates to an active matrix type display device, and in particular to a layout of pixels and wiring in such a device.
2. Description of the Related Art
Electroluminescence (hereinafter referred to simply as “EL”) display devices in which a self-illuminating EL element is used for each pixel have advantages such as thinness and low power consumption, in addition to being self-illuminating. Because of these advantages, EL display devices have currently attracted much attention as an alternative display device for liquid crystal display devices (LCDs) and CRTs, and effort is therefore now being expended on their development.
In particular, an active matrix type EL display device in which a switching element for individually controlling an EL element such as a thin film transistor (TFT) is provided in each pixel so that the EL element is driven individually for each pixel is highly expected as a high-resolution display device.
FIG. 1 shows a circuit structure for a pixel in an active matrix type EL display device having m rows and n columns. In this EL display device, a plurality of gate lines GL extend on a substrate in the row direction and a plurality of data lines DL and power supply lines VL extend on the substrate in the column direction. Each pixel is formed in a region surrounded by gate lines GL and data lines DL, and comprises an organic EL element 50, a switching TFT (first TFT) 10, an EL element driving TFT (second TFT) 20, and a storage capacitor Cs.
The first TFT 10 is connected to a gate line GL and a data line DL and is switched ON based on a gate signal (selection signal) received through a gate electrode. A data signal supplied via the data line DL at this point is stored in the storage capacitor Cs which is connected between the first TFT 10 and the second TFT 20. A voltage corresponding to the data signal supplied through the first TFT 10 is supplied to the gate electrode of the second TFT 20, and the second TFT 20 supplies an electric current corresponding to the voltage value from the power supply line VL to the organic EL element 50. As a result of these operations, light is emitted from the organic EL element 50 of each pixel with a luminance corresponding to the data signal so that a desired image is displayed.
In liquid crystal display devices (LCD) which are currently widely used as flat panel displays, color display is already common. In such color LCDs, each of a plurality of pixels which are placed on a substrate is assigned one of several colors such as, for example, R (red), G (green), and B (blue). Similar color display devices are also desired for display devices using an organic EL element. In order to realize such a color display, the basic arrangement of the R, G, and B pixels is common to the arrangement used in color LCDs.
For example, in a color LCD or the like, typically, a data signal (display signal) is supplied to each color through different data lines for pixels corresponding to each color on the substrate, in order to simplify the display signal processing and the driving circuit and to reduce the influence from display contents for differing colors. A stripe arrangement is known as an arrangement of pixels for a color display device in which pixels of the same color are aligned in the column direction. In active matrix type color LCDs in which such stripe arrangement is employed, the data line for supplying a data signal (display signal) to a thin film transistor which controls a liquid crystal capacitor for each pixel extends in an approximate straight line in the column direction, similar to the case for monochromic displays. Data signals can be supplied through one data line to a plurality of pixels of the same color aligned in a same column.
Similarly, when an active matrix type color organic EL display device is realized with the circuit structure of FIG. 1 and the stripe arrangement, the pixels each having an organic EL element 50 of the same color are aligned in an approximate straight line in the column direction. Therefore, the data lines DL for supplying data signals to each pixel and the driving power supply lines VL for supplying current are both arranged in an approximate straight line in the column direction along the arrangement of the pixels.
On the other hand, an arrangement commonly referred to as a delta arrangement is known as an arrangement of pixels in color display devices for displaying images with higher resolutions, in which the pixels of the same color are arranged in the column direction not in a straight line but with a shift of a predetermined pitch for each row. For active matrix type LCDs, display devices with such delta arrangement are widely known. The pixels of the same color are placed with a shift of, for example, a distance corresponding to 1.5 pixels in the row direction. Therefore, the data line for supplying a data signal to the pixels of the same color extends in the column direction in an undulating manner between pixels that are shifted for each row.
Similarly, in organic EL display devices, it is expected that such delta arrangement will be employed in response to a demand for improving the resolution or the like. However, in an active matrix type organic EL display device, as shown in FIG. 1, a data line DL and a driving power supply line VL must be connected to each pixel in the column direction and, thus, when the delta arrangement is employed the wiring becomes more complicated than in a comparable LCD. In addition, in many cases these two wirings are formed through simultaneous patterning using the same material to combine the manufacturing processes. In this case, it is necessary that these two wirings be placed in the column direction without intersecting each other. Because of the above described reasons, it is desired that at least the data line DL be connected to the pixels of the same color.
FIG. 2 shows a possible example layout of pixels in an active matrix type organic EL display device wherein the delta arrangement is employed. The structure shown in FIG. 2 is designed so that the pixels of the same color which are connected to the same data line DL are symmetric about the data line DL. For example, as shown in FIG. 2, in a pixel for R (“R pixel”) on the first row, the first TFT 10 is placed on the right side in the figure of the pixel and is connected to the data line 43r, and in an R pixel on the second row, the first TFT 10 is placed on the left side in the figure and is connected to the same data line 43r. In this manner, by employing a pattern within the pixels which alternates between left and right for each row, the undulation of the data line 43 among the rows is inhibited to a distance corresponding to one pixel for the case shown in FIG. 2 where the pixels of the same color are shifted by a distance corresponding to two pixels for each row. This structure is employed because the shorter undulating distance results in less problems such as delay and attenuation of the signals or the like caused by the wiring resistance.
On the other hand, the driving power supply line (VL) 45 is connected to a common driving power supply Pvdd and is not required to be connected to pixels of the same color. As described above, however, the driving power supply line VL 45 must not intersect the data line 43, in order to pattern the driving power supply line VL 45 simultaneously with the data line 43 using the same material. Therefore, the driving power supply line VL extending in the region between the R pixel and G pixel in the first row and connected to the second TFT 20 of the G pixel in the first row, as shown in FIG. 2, for example, passes a region between a data line 43g for G and a data line 43r for R, extends in a region between the R pixel and G pixel in the second row, and is connected to the second TFT 20 of the R pixel in the second row.
The layout of FIG. 2 satisfies the conditions that the data line DL is connected to the pixels of the same color with a short wiring length and that the driving power supply line VL is connected to each pixel without intersecting the data line DL. However, as is clear from FIG. 2, the wiring undulates in a complex manner among the rows and the area occupied by the wiring in the region between the rows is large. When the area occupied by the wiring is large as in this case, the emission region (formation region of the organic EL element) is significantly limited on a substrate having a limited area, and, thus, improvements in aperture ratio cannot be achieved, that is, bright display cannot be produced.
In addition, because of the convoluted wiring, the total length of the wiring becomes long, and, consequently, the wiring resistance is also increased. For example, the maximum current value that can be supplied by the driving power supply line 45 must be substantially the same for any pixel at any position of the display device, otherwise a variation in emission brightness will occur among the organic EL elements 50 on a display screen. Therefore, when the wiring resistance of the driving power supply line 45 is increased, there is a problem in that the pixels located at positions farther away from the driving power supply will exhibit lower emission brightness because of the voltage drop caused by the wiring resistance of the longer driving power supply line.